Ferritin forms a 24-meric structure, and stores iron that is a metal essential for living organisms in an internal cavity formed by its structure. Ferritin-like proteins are ubiquitously present in organisms from animals and plants to microorganisms, and profoundly involved in homeostasis of an iron element in the living organisms and cells. One of the ferritin-like proteins that the microorganisms have is called Dps (DNA-binding protein from starved cells). Dps forms a 12-meric structure consisting of a monomer unit having a molecular weight of about 18 kDa, thereby forming a cage-like structure having an external diameter of 9 nm which has an internal cavity with a diameter of about 5 nm, and can store an iron molecule as an iron oxide nanoparticle in this internal cavity. Furthermore, it was shown that ferritin is able to artificially store the nanoparticles including oxides of metals such as beryllium, gallium, manganese, phosphorus, uranium, lead, cobalt, nickel, and chromium, and semiconductors/magnetic substances such as cadmium selenide, zinc sulfide, iron sulfide and cadmium sulfide, in addition to iron. Thus, ferritin is actively studied on its application in semiconductor material engineering and medical fields (Non-patent Literature 1)
In addition, peptides capable of binding to the inorganic material or the organic material are developed by screening using a phage for the purpose of making a complex of a biomaterial and an inorganic material or an organic material. For example, the peptides that recognize carbon nanotube (CNT) and carbon nanohorn (CNH) (Non-patent Literature 2, Patent Literatures 1 and 3), titanium oxide (Patent Literature 2), gold (Non-patent Literature 3), zinc oxide (Non-patent Literature 5), germanium oxide (Non-patent Literature 6), and zinc sulfide and cadmium sulfide (Non-patent Literature 7) and the like are known as such peptides.
Nanographite structures such as CNT and CNH that are carbon crystalline structures are expected to be applied to electronic materials, catalysis, optical materials, medical technology, and the like by constructing a complex with the other nanomaterial based on their electric properties and structures. Technology of constructing a nanocomplex by attaching the nanographite structure with metal nanoparticles using ferritin fused with a CNH binding peptide is reported (Non-patent Literature 2 and Patent Literature 3).
Titanium oxide generates an electron having a reduction capacity and a hole having an oxidation capacity on its surface by light energy when receiving the light. By taking advantage of its oxidation capacity and reduction capacity, titanium oxide is attempted to be applied to antimicrobial materials, deodorant materials, air cleaning materials, anti-stain materials, hydrogen generating catalysts, solar cells, and the like (Non-patent Literature 11). For example, when hydroxide ion is oxidized utilizing the oxidation capacity generated on the surface of titanium oxide by the light, radical having strong oxidation capacity can be generated. The radical can enhance biocidal effects, effects of decomposing odor substances such as acetaldehyde and ammonia, effects of decomposing harmful substances such as NOx and formaldehyde in air, and effects of decomposing dusts by its oxidation capacity. It is also attempted that water is electrolyzed utilizing the generated oxidation reduction capacities to produce oxygen and hydrogen and the hydrogen is utilized as clean energy. Titanium oxide can be utilized as the solar cell by isolating excited electrons generated in titanium oxide by light. Titanium oxide can be functioned as the solar cell by adsorbing a dye as an enhancer to the surface of the titanium oxide and isolating the excited electrons generated by irradiating the dye with light.
In order to enhance a performance of such materials utilizing the titanium oxide, it is conceivable to increase a surface area of the titanium oxide and enhance electric properties of the titanium oxide. Specifically, the increase of the surface area of the titanium oxide can increase total numbers of electrons having reduction capacity and holes having oxidation capacity which are generated by light energy, and a total number of dyes adsorbed to the surface. In addition, the enhancement of the electric properties can decrease a probability that electrons excited by light are bound again to holes. Thus, more electrons and oxidation capacity can be obtained.
To date, technology of laminating an oxide film and nanoparticles by arranging the nanoparticles on a silicon substrate, forming a titanium oxide film or a silicon oxide film on the nanoparticles, and arranging the nanoparticles on the oxide film, which technology utilizes a metal encapsulating protein, ferritin fused with titanium oxide, is known (Patent Literature 4). It is also reported that a protein encapsulating metal nanoparticles of cobalt or an iron oxide is oriented on a pattern depicted by titanium utilizing the metal encapsulating protein, ferritin fused with titanium oxide (Non-patent Literature 8).
Further, it is also reported that a CNT surface is coated with titanium oxide to change the electric property of CNT by using a polypeptide consisting of 35 amino acid residues in which a CNT-binding peptide was fused to a titanium oxide-binding peptide (Non-patent Literature 10). Technology of coating the carbon nanotube with titanium oxide using virus (Non-patent Literature 11) and technology of coating the carbon nanotube with titanium using polyoxometalate (Non-patent Literature 12) is also reported.